This invention relates to a hydraulic driving apparatus using a bladder-type accumulator and, in particular, to an improvement in safety of the bladder-type accumulator.
Generally, a hydraulic driving apparatus is provided with an accumulator. The accumulator is a sort of vessel for receiving hydraulic fluid supplied from a hydraulic fluid source and for accumulating or reserving the hydraulic fluid in a pressurized condition. The accumulator serves to absorb pulsation of a hydraulic pressure and to achieve a quick start of hydraulic driving operation without using a large-scale pump. As the accumulator of the type described, a bladder-type accumulator is well known.
The bladder-type accumulator comprises a pressure vessel and a rubber bag (called a bladder) contained in the pressure vessel. A gas (for example, nitrogen gas) is confined in the bladder at a predetermined pressure. The bladder-type accumulator is connected to a fluid supply pipe between the hydraulic fluid source and an actuator. In the bladder-type accumulator, the bladder is expanded and compressed in response to pressure variation of the hydraulic fluid surrounding the bladder.
In the meanwhile, the bladder-type accumulator of the type described is often damaged due to two factors which will presently be described. In the first place, the hydraulic fluid may leak from anywhere in a hydraulic circuit. In this event, the hydraulic fluid within the pressure vessel is completely discharged out of the pressure vessel. As a consequence, the bladder is rapidly expanded until it is pressed against an internal wall of the pressure vessel. This state is called "zero-pressure down". In the second place, the bladder may excessively be compressed by the hydraulic fluid when the hydraulic fluid has an increased pressure. In this event, the bladder is partially deformed and acutely bent. This state is called "overcompression".
In order to prevent the bladder-type accumulator from being damaged due to the above-mentioned factors, the hydraulic pressure in the pressure vessel must be kept within a predetermined range. To this end, it is proposed to provide the hydraulic fluid source with an unload/onload switching section. The unload/onload switching section serves to adjust a flow rate of the hydraulic fluid supplied from the hydraulic fluid source to the bladder-type accumulator. The unload/onload switching section is controlled by a control unit. The control unit is supplied from a pressure sensor with a pressure detection signal representative of a hydraulic pressure in the fluid supply pipe (preferably in the vicinity of the accumulator).
However, even if the unload/onload switching section is provided as described above, the following disadvantages still remain unsolved.
First, confinement of the gas in the bladder may be insufficient because a gas confinement pressure is confirmed by an operator periodically or on demand. Sometimes, the operator is unaware of decrease of the gas confinement pressure resulting from leakage of the gas. It is noted here that the pressure sensor detects the hydraulic pressure in the fluid supply pipe alone and the gas confinement pressure in the bladder can not be detected. This results in occurrence of the overcompression in the bladder-type accumulator. The overcompression may also be caused when the gas confinement pressure in the bladder is lowered by a decrease of an ambient temperature or a hydraulic fluid temperature. On the contrary, confinement of an excessive amount of the gas in the bladder or an increase of the ambient temperature or the hydraulic fluid temperature causes the zero-pressure down.
Second, an operation of the actuator may require consumption of the hydraulic fluid beyond an allowable range. In this event the hydraulic pressure within the pressure vessel is decreased to cause the zero-pressure down.